Epoxy resins possess good thermal, mechanical, electrical, adhesive, and weather-resistant properties, and, therefore, have been widely used in paints, coating agents, electrical insulation materials, matrix resins for fiber-reinforced products and the like.
However, conventional epoxy resins have poor thermostability at a temperature above 180.degree. C. As the thermostability of an epoxy resin depends on the ability of a curing agent to form a crosslinked three dimensional network, selection of the curing agent is of utmost importance for the production of a thermostable (i.e., heat-resistant) epoxy resin.
Conventional curing agents such as amines and anhydrides have problems in that: lengthy curing time is required; environmentally harmful gases are released during the curing procedure; large amounts thereof are required, thereby increasing the production cost; and the properties of the end products are often sacrificed due to the curing agents, particularly when they are employed in combination with additional agents such as fillers, plasticizers, releasing agents, flame retardants, diluents, dyes and the like.
In order to solve the above problems, extensive R & D efforts have been made to develop, e.g., a heat-latent, cationic polymerization initiator which is normally inactive during storage but is capable of initiating a cationic polymerization reaction on heating to an elevated temperature. This type of polymerization initiator is generally referred to as a heat-latent curing agent.
For example, U.S. Pat. Nos. 5,070,161 and 5,066,722 describe the use of benzylpyridinium or benzylammonium salts of a non-nucleophilic anion; and U.S. Pat. No. 5,132,377 discloses the use of quaternary ammonium salts of non-nucleophilic anions, the quaternary ammonium ion having an .alpha.-substituted benzyl group attached to the quaternary nitrogen atom thereof, as heat-latent, cationic polymerization initiators.
Polymerization initiators disclosed in the above-mentioned references are capable of curing epoxy resins quickly when heated to an elevated temperature (see J. Gu et al., J. Appl. Polym. Sci., 30, 2997-3007 (1985); K. Morio et al., J. Appl. Polym. Sci., 32, 5727-5732 (1986)). However, these heat-latent curing agents have a problem in that resin shrinkage occurs during the process of curing.
Therefore, there has continued to exist a need to develop an effective curing agent capable of providing an epoxy resin having good thermal and dimensional stability.